Modulation



April 27, 1948; e. 1.. USSELMAN 2,440,622

' MODULATION Filed Aug. '2, 1945 2 Sheets-Sheet 1 A 4400001 7/0 Farm mu:

' INVENTOR George L. flue/man BY .Aov-M,

ATTORN EY April 27, 1948. e. 1.. USSELMAN MODULATION Filed Aug. 2, 19452 Sheets-Sheet 2 fi/P/W/VG 0/2 a mar/ma MM:

INVENTOR V George [1. arfelmalz ATTORNEY In both embodiments a pointintermediate the ends of the tank circuit is coupled to the cathodes ofthe tubes and the control grids of the respective tubes are crosscoupled to the tank circuit to operate in the oscillation generatingcircuits approximately 180 out of phase with respect to each other andto the radio frequency voltage on the anodes of the said respectivetubes.

In the embodiment illustrated in Fig. 1 a keyed tripping circuit andadditional modulating circuits are arranged to be coupled to the tubeelec-' may be used if desired. A point centrally located between theends of inductance LI is connected to the positive terminal of a sourceof direct current potential and to ground and the cathodes of tubes VIand V2 by a capacitor CID which bypasses voltages of the generatedfrequency. This point is also connected to a point intermediate thesections of capacitor CI by a resistance R that uncouples the capacitiveand inductive branches of the tank circuit so that split tuning will nottake place. In other words, resistor R prevents the upper and lowerhalves of the tank circuit from being resonant at different frequenciesin case their constants are not similar. The use of R is desirable inorder that all parts of the tuning condenser be kept at the same directcurrent potential and thereby prevents arcing or condenser breakdown. Italso increases the rating of the condenser since it has only to beinsulated for the A. C. voltage on it.

A phase shifting network comprising series capacitors C2 and C3 andshunting resistances R3 and R4 with R5 connect a point on the inductanceLI to the screen grid 8 of. tube VI and the screen grid II] of tubeVZ.These connections include radio frequency bypassing and coupling anddirect current and audio frequency blocking condensers C4 and C5. Asillustrated the tap on LI and the lead to anode 2 are on the same sideof the grounded point on LI. The tap may be moved to the other side ofthe grounded point without-altering the principle of operation. Ra-

' dio frequency bypass condensers C6 and C1 couple the low radiofrequency voltage ends of resistances R4 and R5 to'the cathodes of thetubes. Capacitors C6 and C1 are large' and are used to complete theradio frequency circuits of the phase shifting networks but are smallenough to block currents of the frequency of the modulating potentialsapplied from any desired'means, such as a tripping circuit ortransformer, to-leads I and 3, and thence differentially to the screengrids 8and I'll through resistors R4 and R5. The control grids I4 and I6are cross-coupled by coupling condensers II and I3, to points on LI atopposite sides of the point thereon put at radio frequency groundpotential by condenser CH3. A common cathode biasing resistance RI isshunted by a radio frequency bypassing capacitor An output value. Amodulation source A for telephone may be coupled to a transformer TIprimary winding. The secondary winding of this transformer is coupled topoints on the resistances R8 and R9. A switch S may be included in thiscoupling. The resistors R8 and R9 are grid leak resistances. Thesecondary winding of transformer TI is tapped up on these resistors topoints at which the resistance matches the impedance of transformersecondary Winding.

the

The common cathode resistance Ri maintains substantially constant biason the control grids I4 and I5 of tubes VI and V2 since the sum of thecurrents through both tubes is substantially constant if the'modulatingpotentials are properly balanced. 'Inmy system they are balanced byproper selection of resistances R4 and R5 and condensers C8 and C9 andapplication of proper modulating potentials to leads I and 3.

Consider the system as described as an oscillation generator andassumethe electrodes are energized and tank circuit C ILI tuned to the desiredoperating frequency. Oscillations are generated due to regenerativefeedback between the-anode and control grid in each tube. Note the R; F.voltages are opposed on the anodesreversed and opposed on the controlgrids so that the set up is right for regenerative operation. Each tubeas connected is a regenerative generator. The-two tubes are entrained bythe tank circuit LICI to operate as a single generator operating at acommon frequency.

The screen grids 8 and Ill of tubes VI and V2 are coupled by phaseshifting networks C2C3, R3R4 with R5, to a voltage point on vLI whereatthe radio frequency voltage phase is the same as the phase of thevoltage on the anode 2. The phase of this voltage as applied to the grid8, however, is relatively advanced about 90 by the phase shiftingnetwork. Thepotential on the control grid I4 however is 180 out of phasewith respect to the potential on the anode 2. As a consequence thescreen grid 8 and control gridI4 are excited substantially in phasequadrature relation. The sum or resultant of theexcitation on thecontrol grid I4 and the screen grid 8 of the tube VI is laggingsomething less than 90, say 45 displaced relative to the voltage in thecircuit C ILI, so that the phaseof the anode current output of tube VIis lagging the phase of-the oscillating current in tank circuit C ILI byabout 453 grid-I8 of tube'V2 is the same as that'on the screen grid IQof tube V2 except for the phase advance due to the phase shiftingnetwork. The

'sum of the resultant A. C. voltage on the control grid I6 and thescreen grid I0 is leading, this time something less than 90, say 45,relative to the phase of the current in the tank circuit CILI so thatthe anode current output of tube V2 is say 45 leading this time thephase of the oscillations in the tank circuit CILI The frequency ofoperation of the individual tubes and of the tubes as entrained by thecircuits depends on the phase relation of the radio frequency voltageson the tube electrodes and on the state of conductivity of therespective tubes.

" in my U. SpPatent $2 2,326,314.

"Hill-k6 direct currentipotentialsare appliedtoithe .1 wscreengridsandthe .phase of the electrode volt- 'tages are asndescribedabovethe system will scillate at a trequency depending primarily upon thetuning of circuit CILI.

Theiexcitation energy from theanode 2 end of the tuned circuit to thescreen. grid 18 is relatively advanced 90 by the phase shifting network.'The -excitation energy to the screen grid ill -of' V2 is alsorelatively advanced by its phase Y shifting network but is initially 180out of phase 'with respect to the anode 4 voltage so that theexc'ita'tion on the screen grid it is lagging the anode-4 Voltage byabout-90.

The phase of theoscillations generatedin the tank circuit CILI is theresultant :of the anode -powerdeliveredto it from the tubes 'VI and V2.Similarly the phase of the power delivered by the tubes Vl and V2 is theresultant ofthe phase'of the control grid excitation volt- 4 ages bywayof condensers H and I 3, and the excitation voltages on the screengrids8 and It.

Since the resultant grid excitation in tube V! is -lagging it deliverspower of lagging phase angle tothe tank circuit ClLI. 'In contrast sincethe. resultant grid excitation in tube V2 is leading it delivers powerof leading phase angle'to the tankcircuit C lLl. As long as the tubesVIV2 deliver equal power-as is the case when no modulation ispresent,then the oscillator operates at. "the normal frequency to which CILI aretuned. However, if one tube delivers more power than the other*the'oscillator frequency will change by pulling the frequency. one wayor the other. *For'example, if during the modulation cycle the ofmodulatingz..potentialssuch as those reprewsentingwolce. signals andvthelike. -"When the switch ison contacts- 12 the leads 1';- andBemay 1bezconnected toltheresistors l and or :a tri-pping keying circuit:- suchas I disclosed: more in idetail cin my. Uzi S. application: 'Serial"#521;907,

, filed Eebruaryi 11; 1944. "This keyedrtrippingor switching circuitincluding tubes viarid V4 supplies-at the-leaded!) and 32directcurrentpotentials which are appliedby way of leads I and 3 to-thescreening electrodes-8 and I0; The direct current potentials are made ofa value sufchtas to alternately key the tubes V I and *V2 from a l moreconductive condition to a less conductive or a non-conductive condition."Preferably thepo- 'ten'tials swing from a positive value at which thethe anode of the said one tube td the grid of the other tube. In thismanner currentis caused screen grid of tube Vi is biased more positiveand the screen grid of tube V2 is less positive then since Vl deliversphase retarded power and more power than tube V2 the frequency of theoscillatorwillbe decreased. On the otherhand, if the power output of thetube V2 is increased by-making its screen grid Ill more positive andthepotential of screen-grid 8 of tube Vl is made less positive, sincethe phase of the power 'from gtubeVZ is-leading-the' frequency of theoscillator will be increased. 'Modulation potentialapplieddifferentially to the screen grid electrodes by leads land 3 results infrequency modula- .tion of "the, oscillations generated in accordancewith the modulation potentials. Amplitude variation "of the modulatedradio frequency by the signalsor by power supply variations arecorrected'due to thedifferential action. The modulationpotentials mayrepresenttelegraphy code .orlsimilar-signals or music or voice. Thelat-- ter maybe applied by a transformer secondary winding tapped tosupply the direct current potentials'ior the screen grids or telegraphsignals ,may be applied by control circuits as disclosed Modulationmayalso or. alternativelybe applied from source Ajdifferentially to. thecontrol grids l4 and I6, when switch S is closed.

In Fig. II have shown somewhat completely modulating and keying meanswhich may be used .with theoscillation generators of Figs. 1 and 2.Switches SI and S2 are used to switch the modu- .lator to the ffphonesource, contacts a, through ,transformerTz or to thetripping circuittubes V3 and iVdcontacts. b. A double pole triple throw Qswitch may beused for this purpose. The leads to flow through tube-V3 while tube V4iscut off or vice versa.

A point intermediate the resistances 43 and i4 is connected to thepositive terminal of a source of direct current potential. A point.intermediate the terminals of resistances 45 and 6 is connected to apoint-on a-potentiometer resistance' la shunting a source ofpotenti'alB2 the positive terminal of which is grounded. "This source ofpotential is shunted by a condenser for potentials of the keyingfrequency. *The resistances "45 and 46 whichare potentiometers havepoints thereon connected to theleads fill and 32and thence by leadsland-3 to the screening electrodes of tubesVl and V2 for mod-ulationpurposes. The outer ends of the resistances 45 and Marc connected toan'alterna'ting varyingpositive voltage source, i. e., to the anodes oftubes V3 and V4 which, as described' hereinaft'er,. swing alternatelypositive and less positive as the control gri'ds of the tubes-or-0nethereofis 'controlledby signal potentials. "The potential-on the leads39 and'32 therefore is a resultant-of the potentials on the anodesoftubes V3 andVd andof the potentialfrom the main direct current source DCand the potential de- *rived from source B2 by way of the potentiometer49 and also of and V4.

The tripping tubes V3 and V4 have their control gridscoupled'respectively by resistances 58 the conductivityof the tubes V3and filr'and' 59 and'63, to ground; andbyreon resistance 6| connected byswitchS3 either tothe positive terminal of a source B3 or to ground. A'point on potentiometer resistance-:63 is connected to a source ofsignals, forexample, keyed pulse energy, or keyed direct current orkeyed alternating current energy. The-ot=her trol grid oftubeV4"wit-h-respect to its cathode.

' on contact KI.

7 The source B3 is sometimes necessary to apply a fixed negative bias tothe grid of tube V3 in order to get positive tripping action. The ar-'rangement is suchthat the tube V3 is biased to outofl in the absence ofmarking and spacing signals either by the drop in potential inresistances 58 and 6| or if necessary by negative potential from thesource B3 with switch S3 on contact KI instead of on K2. The currentthen is switched through tube V4 and the resistances 43, 44, 45 and 46,the direct current source and the tap on potentiometer 49, etc., aresuch that the potential at the anode of V4 drops the required amount.This falling potential is fed to the screen grid I by leads 32 and 3 andis suflicient to swing the screen grid I0 negative or less positiveenough to block or bias tube V2 and cut off or reduce currenttherethrough. At the same time the anode of tube V3 swings more positivebecause current through tube V3 is cut oil or reduced, and its anode isat or near the direct current source potential. The elements aredimensioned and adjusted to values such that the screen grid for .tubeVI is made positive by this positive potential supplied by leads 30 andI, and tube VI draws current so that oscillations of a lagging phase aresupplied by tube VI to the tank circuit and the frequency of operationof the system goes down and oscillations of one frequency aretransmitted. They might well represent no signal or space signal.

Now when keying signals are applied an alternating current or directcurrent potential fed at the input and applied to the grid of tube V4switches the current through tube V3 by blocking or biasing tube V4 andby virtue of the cross-couplings 42 and II. With the current switchedthrough tube V3 and cut off in tube V4 a more negative potential is fedby leads 30 and I to the screen grid 8 of tube VI and a more positivepotential is applied from the anode of tube V4 by leads 32 and 3 to thescreen grid I0 of tube V2 and this tube V2 becomes conductive to supplyoscillations in the tank circuit LICI of leading phase so that thefrequency of the oscillations becomes higher and oscillations of thisfrequency may represent the marking condition.

' Alternating current or direct current may be used for keying purposes.If alternating current pulses are applied to the control grid of tubeV4, through which the current is switched in the absence of signals, thepositive excursions of the alternating current have no effect becausethe tube V4 is already drawing maximum current. The negative cycles ofthe keyed alternating current however, make the grid of tube V4 negativeto cut oif current therein and switch the same through tube V3 asdescribed hereinbefore. Adjustment of the point on potentiometer BIpermits operation such that the positive bias on the tube V4 is overcomeby the applied'signals to insure the current switching action. Ifnecessary more negative potential may be applied to the grid of V3 fromthe source B3 by putting the switch S3 1 have found that when telephonysignals or tone modulation is applied to the screen grids of amodulator, distortion in the modulated outputmay result. In Fig. 1 Ishow a circuit arrangement by which the signal modulation may be appliedto the control grids of the modulator tubes as well as to the screengrids. In this embodiment phone signals or tone signals may be appliedto the control grids and telegraphy or phone or tone signals maybeapplied to the potentials are then superposed on the positive bias andapplied from source B differentially by way of leads I and 3 to thescreening electrodes 8 and Ill. The modulating potentials may representtone or phone signals. During this operation and when the switches SIand S2 are on the b contacts the switch S is open to disconnect thesource A from the control grids of the tubes VI and V2.

When telephony signals or tone modulation is applied to the screen grids8 and III of my system some distortion in the modulated output may 7result.

In Fig. 1 I show a circuit arrangement by which the signal modulationmay be applied to the control grids of the modulator tubes instead of tothe screen grids. In this embodiment phone signals or tone signals maybe applied to the control grids from source A by way of transformer TIand resistances R8 and R9 when the switch S is closed. Application ofthe telephony or tone modulation to the control grids producessubstantially undistorted voice or sine wave modulation of the frequencyof the output in accordance with the signals. Then the switches SI andS2 may be placed on the 0 contacts to supply an adjustable D. C.potential to the screen grids 8 and I0.

If desired tone or phone modulation from source A may take placesimultaneously with telegraphy modulation by signals applied to theleads 49 and 5 I. To do this the switch S is closed as stated above andthe switches SI and S2 are moved to the 1) contacts so that the outputof the tone keyer is applied to the screen grids while the output of thephone or signal source A is applied to the control grids.

I have shown an improved signalling system of this type in Fig. 2. InFig. 2 I have used reference numeral corresponding to those used inFig. 1. The arrangement in general'is as shown in Fig. 1. The trippingcircuit is shown by a rectangle but the details there may besubstantially as in Fig. 1. I

The source B2 is omitted in the practical arrangement illustrated inFig. 2 and the potentiometer resistance 49 shunted by condenser CIII nowalso is in shunt to the main direct current source.

The radio frequency circuits are substantially similar to those inFig. 1. However, in the arrangernent of Fig. 2 I have made theoscillation generator more stable so that its average or mean frequencyis more constant in spite of voltage changes and ambient temperaturechanges. To

do this I include resistors R.I3 and RI 4 in the anode leads ,to thetank circuit. Moreover, to further enhance stability the oscillator tankcircuit CILI is built with a relatively large capacitive reactance CIand low inductive reactance LI. The condenser CI and the coil LI areboth constructed of materials and methods resulting in low coefiicientsof expansion. Moreover, the tank circuit LICI is enclosed in atemperature control boxv TCB, the interior of which isheldzata.substantiallyiconstant temperature byimeansiknown inxthe. art-but notshown here. Thea'ends of:;th,e tank. circuits-are as statedconnecteditotheanodes sand 4 by-the: damping resistors. Hi3 and Bit. :Tofurther improveqthe operating characteristics. the tank circuit =-LI iscoupled to the. control gridlfi of an electron discharge device Vthrough resistor 16 and coupling condenser 78. 82 is the grid leakresistance and St is the cathode return resistance shunted by condenser85. Anode potential is supplied from the main source through resistance81. The output of the system is then taken from the anode and cathode ofthe stage V5 by way of ground and condenser 89.

The constant temperature at which the tank circuit GIL! is held holdsthe capacity inductance thereof substantially constant. The resistancesRI3 and RM and 16 act to minimize any effect of the tubes VI, V2 and V5on the average frequency of the oscillations generated in the tankcircuit due to temperature changes in the tubes and to changes in tubecharacteristics when tubes are changed. It has been found thatconnecting a resistance across a parallel tuned circuit does not changeits frequency response. How ever, if a reactance, inductive orcapacitive, is connected across a parallel tuned circuit then itsfrequency response is changed. The anodes of tubes VI and V2 present aconsiderable amount of capacitive reactance when connected across thetank circuit as shown in Fig. 1. In this case a change of tubes or achange of tube temperature will change the anode to suppressor gridcapacity and consequently it will change the oscillator tank circuittuning. This, of course, changes the oscillator frequency. Whenresistors Rl3 and RM are connected between the tank circuit and theanodes of VI and V2 as shown in Fig. 2, a variation of tube anodecapacity will not change the tank circuit tuning and the oscillatorfrequency to such an extent. The larger resistors R43 and Rid are madethe less the tube capacity will aifect the oscillator frequency. Itwould be advisable to make Rt 3 and RM as large as possible and stillmaintain oscillations in the oscillator tank circuit. The tube gridconnections are usually made quite far down on the coil toward thecenter point and therefore do not materially affect the oscillatorfrequency. The anodes are usually connected to the ends or near the endsof the tank circuit and consequently they have a much greater effect onthe oscillator frequency. Also, the capacities of the grids are usuallymuch less than that of the anodes. It can also be said that the greaterthe impedance (resistance and reactance) that is connected across aparallel tuned circuit the less effect it will have on the tunin of thetank circuit.

The resistances RI 3 and El 4 also will cut down the amount of frequencydeviation possible. However, this deviation is considered ample anywayso that it is desirable to sacrifice some of it in the interest of anaverage constant frequency. It may also be noted that the resistor 16 istapped on inductance Ll at a point equally opposite the tapping point ofcondenser C2 (in the phase shift network) so that the two approximatelybalance their effect on LI. The output tube V5 has been added to lightenthe load on the tank circuit CILI to thereby aid in holding a moreconstant aver- 10 and RI 4 should ba made; as: large as. practicalandiistillmavesatisfactory operation, ofv thefrequency modulatorcircuit: .o'luring:- modulation.

The source A andthe tripping circuit; are con- 1; nested and, operatesubstantially :as: described above. The; modulation source Bahowever: isomitted. Withthe switches .Sl andrsii' inthe position shown, i. e., oncontacts 0 direct current biasing and operating potentials only areapplied to the screening electrodes. The switch S may then be closed anddifferential modulation of the control grid potentials takes place tomodulate the frequency of the oscillations generated as described aboveby making the tube VI say with the phase lagging excitation on thecontrol grid and screen grid predominate when the modulation swings inone direction, and to make the other tube V2 with the sum of the voltageon the screen grid and control grid of leading phase predominate whenthe modulation swings in the other direction.

With switches SI and S2 moved on the b contacts the tripping circuit isconnected to the leads 3 and i and thence to screen grids l0 and 8 tomodulate the generated oscillations in accordance with telegraphysignals as described hereinbefore. Then the output comprises telegraphymodulated oscillations which may also be modulated by tone signals fromsource A. With the switches S open telegraphy signals only are developedby the keying and tripping circuit.

What I claim is:

1. In apparatus for generating and modulating oscillatory energy, twoelectron tubes each having an anode, a cathode, a control grid and anauxiliary grid, a tank circuit parallel tuned substantially to thedesired operating frequency, means coupling each of said tubes and tankcircuit in a regenerative oscillation generator including said tankcircuit coupling the anodes and cathodes of said tubes in pushpullrelation and coupling the control grids and cathodes of said tubes inpushpull relation, the arrangement being such that the phases of thevoltages of the regenerated frequency on the anode of each tube aredisplaced in phase about 180 degrees with respect to the phase of thegenerated voltages on the grid of the same tube, additional couplingsbetween said tank circuit and said auxiliary electrodes such that whenoscillatory energy of the desired operating frequency is developed insaid tubes and tank circuit the phases of the voltages of the generatedfrequency on the control grid and auxiliary grid of each tube aredisplaced by substantially and means for differentially modulating thegains of said tubes in accordance with signals.

2. Apparatus as recited in claim 1 wherein impedances are interposed inthe coupling between the tank circuit and the anode of each tube.

3. In apparatus for generating and frequency modulating oscillatoryenergy, two electron tubes each having an anode, a cathode, a controlgrid and an auxiliary grid, a tank circuit parallel tuned substantiallyto the desired operating frequency, means including said tank circuitcoupling the anodes, cathodes and control grids in push pull relation,said .push pull couplings to the control grids being crossed relative tosaid push pull couplings to said anodes so that when oscillatory energyof the desired operating frequency is developed in said tubes and tankcircuit the phase of the voltages of the generated frequency on theanode of each tube is substantially out of phase relative to the phaseof 11 the voltage on the grid of each tube to provide regeneration, aphase shifting network coupling said tank circuit to said auxiliaryelectrodes so that when oscillatory energy of the desired operatingfrequency'is developed in said tubes and tank circuit the phases of thevoltages of the generated frequency on the control grid andauxiliarygrid of each tube are displaced by-substantially 90", an outputcircuit coupled to said

